The present invention relates to non-dispersive infrared analyzers and is directed more particularly to non-dispersive infrared analyzers having improved infrared source and detecting assemblies.
Instruments which measure the concentration of a component of interest of a sample gas often operate by measuring the quantity of infrared radiation that is absorbed by the sample gas as the latter flows through a sample cell that is irradiated by a beam of infrared radiation of known intensity. By measuring the amount of radiation that is transmitted through the sample cell at wavelengths that are characteristic of the component of interest, the amount of radiation that is absorbed by the sample gas at such wavelengths, and therefore the concentration of the component of interest, may be determined. This transmitted radiation may be measured by any of a number of different types of infrared detecting elements such as thermistors, Luft-type detectors, etc.
Infrared analyzers of the above type are classified on the basis of the numbers and types of sources, gas containing cells and infrared detecting elements used therein. Examples of analyzers which use a single source, a single gas containing cell, and a plurality of infrared detecting elements are described in U.S. Pat. No. 3,920,993, issued on Nov. 18, 1975 in the name of Cederstrand et al., and in U.S. Pat. No. 4,320,297, issued on Mar. 16, 1982 in the name of Cederstrand et al. An example of an analyzer which uses two infrared sources, two gas-containing cells and a single infrared detecting element is described in U.S. Pat. No. 4,355,233, issued on Oct. 19, 1982 in the name of Warnke et al. An analyzer which makes use of a single infrared source, two gas containing cells and one or more infrared detecting elements is described in U.S. Pat. No. 4,467,213 issued Aug. 21, 1984 in the name of C. A. Farren.
In order to assure high sensitivity and a high signal-to-noise ratio, non-dispersive infrared analyzers are designed so that as much infrared radiation as possible is transmitted from the source to the detecting element. In order to accomplish this, such an analyzer is often provided with a source assembly having a parabolic reflector with an infrared heating element located as nearly as possible at the focal point thereof, and with a sample cell having highly reflective interior walls. In addition, if the analyzer uses solid-state detecting elements, it is often provided with a detecting assembly having a parabolic reflector for focusing the radiation that emerges from the sample cell onto one or more infrared detecting elements that are located as nearly as possible at the focal point thereof.
While an analyzer of the above-described type is adequate for many applications, it has limitations which prevent it from realizing its full potential sensitivity and stability. One of these is that the infrared heating element used therein is not sufficiently small and point-like to assure that its source assembly emits a well collimated beam of radiation. As a result, a substantial fraction of the radiation that is emitted by the source assembly either does not enter the gas containing cell or is transmitted through that cell only after multiple reflections from the inner surface thereof. These multiple reflections, in turn, cause the overall quantity of radiation that is transmitted through the cell to be strongly affected by the presence of dirt on the inner surface thereof. This result is highly undesirable because it causes the output signal of the detecting element to drift with time as the flow of the sample gas deposits dirt on the inner surface of the cell.
Another limitation of analyzers of the above-described type is that the non-collimated or off-axis component of the infrared beam (i.e., the component of the beam which is not parallel to the optical axis of the analyzer) is not properly focused on the detecting element by the parabolic reflector of the detecting assembly. This occurs because the off-axis component of the beam does not strike the surface of the reflector at the proper angle for reflection to the focal point thereof. The total amount of radiation that is received by the detecting element is further reduced by the blocking effect of the structures which are used to mount the detecting element.
In summary, non-dispersive infrared analyzers of types that were used prior to the present invention had low sensitivities and stabilities which resulted from the use of inefficient source, sample cell and detecting assemblies.